Combined energy storage and fuel generation with reversible fuel cells

ABSTRACT

An electrochemical system includes a reversible fuel cell system which generates electrical energy and reactant product from fuel and oxidizer in a fuel cell mode and which generates the fuel and oxidant from the reactant product and the electrical energy in an electrolysis mode. The system also includes a reactant product delivery device which is adapted to supply the reactant product to the reversible fuel cell system operating in the electrolysis mode, in addition to or instead of the reactant product generated by the reversible fuel cell system in the fuel cell mode, and a fuel removal device which is adapted to remove the fuel generated by the reversible fuel cell system operating in the electrolysis mode from the electrochemical system.

BACKGROUND OF THE INVENTION

The present invention is generally directed to fuel cells and morespecifically to reversible fuel cells and their operation.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. There are classesof fuel cells that also allow reversed operation, such that oxidizedfuel can be reduced back to unoxidized fuel using electrical energy asan input. The ability to generate electricity and regenerate fuel makesthese fuel cells suitable for electrical energy storage.

BRIEF SUMMARY OF THE INVENTION

One preferred aspect of the present invention provides anelectrochemical system which includes a reversible fuel cell systemwhich generates electrical energy and reactant product from fuel andoxidizer in a fuel cell mode and which generates the fuel and oxidantfrom the reactant product and the electrical energy in an electrolysismode. The system also includes a reactant product delivery device whichis adapted to supply the reactant product to the reversible fuel cellsystem operating in the electrolysis mode, in addition to or instead ofthe reactant product generated by the reversible fuel cell system in thefuel cell mode, and a fuel removal device which is adapted to remove thefuel generated by the reversible fuel cell system operating in theelectrolysis mode from the electrochemical system.

Another preferred aspect of the present invention provides anelectrochemical system, comprising a first means for cyclicallyoperating in a fuel cell mode to generate electrical energy and reactantproduct from fuel and oxidizer and in an electrolysis mode to generatethe fuel and oxidant from the reactant product and the electricalenergy. The system also comprises a second means for providing excessreactant product to the first means operating in the electrolysis modefrom outside the electrochemical system, in addition to or instead ofthe reactant product generated by the first means in the fuel cell mode,such that fuel in excess of fuel required to operate the first means inthe fuel cell mode is generated in the electrolysis mode over apredetermined number of operating cycles, and a third means for removingthe excess fuel generated by the first means operating in theelectrolysis mode from the electrochemical system.

Another preferred aspect of the present invention provides a method ofoperating an electrochemical system containing a reversible fuel cellsystem, comprising cyclically operating the reversible fuel cell systemin a fuel cell mode to generate electrical energy and reactant productfrom fuel and oxidizer and in an electrolysis mode to generate the fueland oxidant from the reactant product and the electrical energy. Themethod also comprises providing excess reactant product to thereversible fuel cell system operating in the electrolysis mode fromoutside the reversible fuel cell system, in addition to or instead ofthe reactant product generated by the reversible fuel cell system in thefuel cell mode, such that fuel in excess of fuel required to operate thereversible fuel cell system in the fuel cell mode is generated in theelectrolysis mode over a predetermined number of operating cycles, andremoving the excess fuel generated by the reversible fuel cell systemoperating in the electrolysis mode from the electrochemical system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an electrical energy storage system.

FIG. 2 is a schematic of an electrical energy storage system with areversible fuel cell system.

FIG. 3 is a schematic of an electrical energy storage system with areversible fuel cell system, which can also generate fuel for useoutside the electrical energy storage system.

FIG. 4 is a schematic cross section of a single SORFC operating in theelectrolysis mode according to a preferred embodiment of the presentinvention.

FIG. 5 is a schematic cross section of a single SORFC operating in thefuel cell mode according to a preferred embodiment of the presentinvention.

FIG. 6 is a schematic side of view of a Sabatier reactor according to apreferred embodiment of the present invention.

FIG. 7 is a system schematic of the major SORFC components operating inthe fuel cell mode, according to a preferred embodiment of the presentinvention.

FIG. 8 is a system schematic of the major SORFC components operating inthe electrolysis mode, according to a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present inventors have realized that regenerative or reversibleoperation of fuel cells can be applied beyond mere energy storage toproduce fuel for uses outside the fuel cell system. A reversible fuelcell is used to reversibly store electrical energy. When electricalenergy is needed from the storage system, the fuel cell operates in afuel cell or discharge mode. In this mode, fuel is oxidized in the fuelcell, electricity is generated, and part or all of the reactant productis stored, if desired. The system is then recharged in an electrolysisor charge mode. In this mode, the system is recharged by supplyingelectrical power to the fuel cell, electrolyzing the stored and/orsupplied reactant product, thereby regenerating the fuel. Theregenerated fuel and optionally the regenerated oxidant are stored andavailable for energy generation in the fuel cell mode. The systemcyclically or alternatively switches operation between the fuel cell andelectrolysis modes for any suitable number of cycles. If more electricalenergy and reactant product than needed to regenerate the fuel issupplied to the reversible fuel cell over a predetermined number ofoperating cycles, then excess or additional fuel can be generated duringthe electrolysis mode during some or all of these cycles. In otherwords, more fuel is generated when the system operates in theelectrolysis mode than the fuel needed to operate the system in the fuelcell mode. This excess fuel can be used outside the energy storagesystem.

FIG. 1 shows an energy source 100 connected via a conduit 110 to anenergy storage system 120. The energy storage system 120 is connectedvia conduit 130 to the energy consumer 140. The energy storage system120 supplies the required power to the energy consumer 140 at all timesor at predetermined times, if desired. The energy supplied to theconsumer comes from the energy source 100, or from the energy storagesystem 120, or a combination of the two. The system shown in FIG. 1decouples the amount of power drawn by the energy consumer 140 from theamount of energy drawn from the energy source 100.

Examples of the energy source 100 include, but are not limited to theelectrical grid, electrical generators and renewable energy sources.Preferred renewable energy sources include photovoltaic sources, such assolar cell arrays, wind power sources, such as wind turbines, tidalpower sources in which power is generated from forces of ocean, sea orlake tides, and geothermal power sources in which power is generatedfrom geothermal heat.

The energy storage system 120 is preferably a reversible or regenerativefuel cell system, as will be described in more detail below with respectto FIG. 2. Examples of the energy consumer 140 include, but are notlimited to residential households, commercial building, such asfactories, hospitals and office building, electrical subgrids, andremote transmitters.

The system 120 will draw more power from the energy source 100 than whatis supplied to the energy consumer 140 in the electrolysis mode and theadditional power is stored in the form of regenerated fuel. In the fuelcell mode the system 120 provides electrical power or energy to theconsumer 140 instead of or in addition to the power provided to theconsumer 140 from the energy source 100. As used herein, the terms“electrical power” and “electrical energy” refer to features ofelectricity provided by the energy source 100 and to features ofelectricity provided to the consumer 140.

FIG. 2 shows details of the energy storage system 120. The system 120 ispreferably an electrochemical system which contains a power managementsystem 160, a reversible fuel cell system 190, a fuel storage device 210and an optional reactant product storage device 230. The system alsocontains electrical connection conduits or wires 150, 170 and 180, aswell as a fuel conduit 200 and a reactant product conduit 220 whichallow the fuel and reactant product to pass between the reversible fuelcell system 190 and the fuel storage 210 and product storage 230devices, respectively.

The power management system 160 may be any suitable controller device,such as a computer or microprocessor, and preferably contains logiccircuitry which decide how to route the power streams. Energy from theelectrical energy source 100 can be directed fully to the electricalenergy consumer 140, fully to the reversible fuel cell system 190, orcan be partitioned between the electrical energy consumer 140 and thereversible fuel cell system 190. It is also possible to feed electricalenergy back towards the electrical energy source 100, which can beapplied for example where the electrical energy source 100 is theelectrical grid. The power management system 160 also controls fromwhere power is supplied to the electrical energy consumer 140. Power canbe supplied from the electrical energy source 100, the reversible fuelcell system 190, or a combination thereof.

The reversible fuel cell system 190 may comprise one or more reversiblefuel cells or one or more fuel cell/electrolyzer pairs. A reversiblefuel cell is a single electrochemical device which generates reactantproduct and electrical energy or power in the fuel cell mode and whichgenerates fuel from reactant product and electrical energy in theelectrolysis mode. A fuel cell/electrolyzer pair includes two separatedevices, where a non-reversible fuel cell device generates energy, andthe other electrolyzer device regenerates fuel.

Examples of the fuel cells and electrolyzers include but are not limitedto high temperature fuel cells, such as solid oxide fuel cells (SOFC),and also include proton exchange membrane fuel cells (PEM). Reversibleor regenerative SOFC's (SORFCs) are well suited to combine energygeneration and fuel regeneration in a single device. However, other fuelcells, such as molten carbonate fuel cells, may also be used in thesystem 190. Preferably, the system 190 contains at least one stack ofreversible fuel cells or stacks of fuel cell/electrolyzer pairs.

The fuel and reactant product storage devices 210 and 230 may compriseany suitable gas, liquid or solid storage devices. Preferably, thesedevices 210, 230 comprise gas or liquid tanks which are opened andclosed with a valve.

The system 120 operates as follows. Power from the electrical energysource 100 enters through conduit 110 and conduit 150 to the powermanagement system 160. When the reversible fuel cell system 190 operatesin the fuel cell mode and provides electrical energy, fuel is fed fromthe fuel storage device 210 to the reversible fuel cell system 190 viathe fuel conduit 200. In the reversible fuel cell system 190, thechemical energy of the fuel is converted to electrical energy, which isthen supplied to the power management system 160. Optionally, all orpart of the reactant product from the reversible fuel cell system aretransferred to the product storage device 230 via the reactant productconduit 220. Products that are not stored are released.

When the reversible fuel cell system 190 operates in the electrolysismode, electrical energy from the power management system 160 is providedto system 190 and the reactant product from the product storage device230 and/or from outside the electrochemical energy storage system 120 isprovided via the reactant product conduit 220 to the system 190. Thefuel is regenerated in the reversible fuel cell system 190 and providedto the fuel storage 210 via the fuel conduit 200.

Any suitable fuels, oxidizers and reactant product may be used. Onepreferred example for the fuel is hydrogen which is reacted with oxygenfrom ambient air to produce a water reactant product. However, otherfuels and oxidants can be used. For example, a hydrocarbon gas, such asmethane, may be used as a fuel to produce water and carbon dioxidereactant product. Other hydrocarbon gases, such as natural gas, propane,hexane, etc., may also be used as fuel. Furthermore, these hydrocarbonmaterials may be reformed into a carbon containing fuel, such as carbonmonoxide, or previously supplied carbon monoxide may also be used asfuel.

If surplus energy is available from the electrical energy source 100 andan excess reactant product is supplied to the reversible fuel cellsystem 190, then the system 190 can generate more fuel than what isneeded by the system 190 in the fuel cell mode. This excess fuel may beremoved from the system 120 and provided for any suitable end useoutside the system 120.

FIG. 3 illustrates the system 120 when the excess reactant product issupplied to the system 120 and the excess fuel is removed from thesystem. The system 120 is identical to the system 120 illustrated inFIG. 2 with the exception of the fuel removal device 240 and thereactant product delivery device 250. Excess fuel is provided out of thesystem 120 through the fuel removal device 240, while the excessreactant product is replenished through reactant product delivery device250. The devices 240 and 250 may comprise any suitable devices which maydeliver fuel and reactant product.

For example, the fuel removal device 240 may be a gas or liquid conduitsuch as a pipe or hose which delivers fuel, such as hydrogen or methane,from the reversible fuel cell system 190 and/or from the fuel storagedevice 210 outside the system 120. Alternatively, the device 240 maycomprise a movable gas or liquid storage container, such as a gas orliquid tank, which is physically removed from the system 120 after thecontainer is filled with fuel. If the device 240 comprises a container,then the device 240 may be used as both the fuel storage device 210while it remains in the system 120, and as a fuel removal device 240,when it is removed from the system 120.

The reactant product delivery device 250 may be one or more gas orliquid conduits which deliver reactant product, such as water and/orcarbon dioxide to the system 120. For example, the device may comprise awater pipe or hose which delivers water to the product storage device230 or directly to the reversible fuel cell system 190. The device 250may comprise two conduits when the reactant product comprises twoseparate components, such as water and carbon dioxide. Alternatively,the device 250 may comprise a movable gas or liquid storage container,such as a gas or liquid tank, which is physically delivered to thesystem 120 filled with reactant product. If the device 250 comprises acontainer, then the device 250 may be used as both the reactant productstorage device 230 while it collects reactant product during the fuelcell mode, and as a reactant product delivery device 250 when itprovides reactant products during the electrolysis mode.

The reactant product delivery device 250 is adapted to supply excessreactant product to the reversible fuel cell system 190 operating in theelectrolysis mode, in addition to or instead of the reactant productgenerated by the reversible fuel cell system in the fuel cell mode. Inother words, the device 250 supplies the reactant product 250 in excessof the amount generated by the reversible fuel cell system 190 operatingin the fuel cell mode. In one example, if the reactant product is water,then the water generated by the system 190 in the fuel cell mode isstored in the storage device 230 and the device 250 provides additionalwater to the device 230. In another example, the water generated by thesystem 190 in the fuel cell mode is discarded and the 250 provides waterto the device 230 and/or to the system 190 in excess of the amountproduced by the system 190 in the fuel cell mode.

In either example, the excess reactant product allows the system 190 togenerate an amount of fuel in the electrolysis mode in excess of theamount of fuel required to operate the system 190 in the fuel cell modeover a predetermined number of operating cycles. Thus, while anomalies,such as breakdowns, early shut downs, etc., may occur during individualfuel cell mode and electrolysis mode cycles, when the system 190 isoperated cyclically over a predetermined number of such cycles, such asover 10 cycles, for example over 100 to 1,000 cycles, the system 190produces the excess fuel.

The reversible fuel cell system 190 may generate more fuel in theelectrolysis mode than it consumes in the fuel cell mode over thepredetermined number of cycles under several conditions. In a firstpreferred embodiment of the present invention, the reversible fuel cellsystem operates at a higher current level in the electrolysis mode thanin the fuel cell mode over the predetermined number of cycles.

In a preferred aspect of the present invention, the reversible fuel cellsystem 190 includes a SORFC stack. It is desirable for the energystorage system 120 to operate at high round-trip efficiencies, which isthe ratio of energy provided to the electrical energy consumer 140 tothe electrical energy from source 100. In order to achieve highround-trip efficiencies, the SORFC is preferably operated at comparablylow current densities thereby minimizing losses in the SORFC.Theoretically, the round-trip efficiency increases with decreasingcurrent density. However, the SORFC should be maintained at an elevatedoperating temperature, which typically ranges between 600° C. and 1000°C. The losses within the SORFC can be used to provide make-up heat tokeep the SORFC at the desired temperature. If the current and therebythe losses become too small, the desired operating temperature cannot bemaintained. Therefore, a practical low limit for the current densityexists in the electrolysis mode and in the fuel cell mode. This currentdensity limit depends on the thermal losses from the system and the heatgenerated at a specific current.

Operation of a fuel cell at low current implies a fuel cell voltageclose to the open circuit voltage (OCV), which is the fuel cell voltageat zero current. Operation below OCV implies power generation, whileoperation above OCV implies fuel regeneration/electrolysis. When thefuel cell is operated in the electrolysis mode, additional constraintson the minimum allowable current density can exist. In the example ofwater electrolysis at high temperature, operation of the fuel cell onlyslightly above OCV is an endothermic reaction. There is a minimumvoltage for electrolysis, termed the thermal neutral voltage at whichthe electrolysis reaction is thermally neutral. In order to thermallysustain the SORFC without the use of thermal storage duringelectrolysis, electrolysis should be performed above the thermal neutralvoltage. On the other hand, power generation can be performed very closeto OCV. As a result the desired minimum current density for electrolysisis significantly larger than the desired minimum current for powergeneration in the fuel cell mode. The difference between the minimumcurrents depends on the thermal management system and the choice of fueland oxidizer. In a SORFC without additional fuel generation, the currentin power generation and electrolysis mode also should be balanced withrespect to the available time for power generation and electrolysis. If,for example, energy is generated for 16 hours per day while fuel isregenerated for 8 hours per day, the current during regeneration has tobe twice as large as the power generation current in order to balancefuel consumed and regenerated.

The two constraints for current densities outlined above generally leadto different values for the minimum current density in the fuel cell andelectrolysis modes. As a result of the higher current density in theelectrolysis than in fuel cell mode, the energy storage system 120typically has additional/unused fuel regeneration capacity. Ifadditional or excess power or electrical energy and reactant product isavailable during fuel regeneration in the electrolysis mode, this excessfuel generation capacity can be utilized to generate additional fuel,which can be used outside the electrical energy storage system.

Thus, the system 120 may be used as a regenerative power supply toremote residences or commercial operations or to residences orcommercial operations connected to the power grid. The electrical powergenerator has to be sized to meet the peak load of the user. This sizingrequirement allows a surplus of fuel to be generated during chargetimes. Examples include, but are not limited to systems sized to meethigh electrical power peak demands which occur during less than 100percent of the system operation in the fuel cell mode or systems withrelatively low energy consumption (i.e., below the maximum possibleenergy consumption) during the fuel cell mode.

Thus, in one preferred aspect of the first embodiment, the reversiblefuel system 190 is operated at a load lower than a peak load that thereversible fuel cell system is capable of providing at least during aportion of a time that the reversible fuel cell system operates in thefuel cell mode. Thus, the reversible fuel system 190 is operated for atleast a portion of its fuel cell mode at a current density below itspeak or maximum possible current density. In contrast, the reversiblefuel system 190 is operated at a higher current density, such as at apeak current density, in the electrolysis mode to produce the excessfuel.

Since the systems 120, 190 are designed and sized for a desired energystorage, the size of the fuel cell stack and the size of the balance ofplant are based on the desired energy storage. Thus, the size of thefuel cell stack is based on the minimum number of fuel cells thatoperate in the fuel cell mode to provide the desired peak power, ratherthan on the number of fuel cells or electrolyzers that are required forthe electrolysis mode. Thus, extra fuel cells or stacks may be requiredensure that the system 120 provides the desired peak power in the fuelcell mode during a worst case scenario. However, the energy generationcapability of all of the fuel cells may not be used in the fuel cellmode outside of the worst case scenario. In contrast, the fuelregeneration capability of all fuel cells may be used duringelectrolysis mode to regenerate the fuel needed for future fuel cellmode(s) and to generate excess fuel for use outside the system 120.

In another preferred aspect of the first embodiment, the reversible fuelcell system 190 is electrically connected to an electrical energy source100 which comprises a photovoltaic energy generation system whichprovides electrical energy to the reversible fuel cell system duringdaytime to generate and store fuel. The photovoltaic energy generationsystem also provides electrical energy to the consumer 140, asillustrated in FIG. 1. The reversible fuel cell system 190 generateselectrical energy during night time from the stored fuel and providesthis electrical energy to the consumer 140 during night time. Often, thenight time load on the reversible fuel cell system 190 is lower than apeak load that the reversible fuel cell system is capable of providing,at least during a portion of the night time period. Thus, the reversiblefuel system 190 is operated in the fuel cell mode during at least aportion of the night time period at a current density below its peakcurrent density. In contrast, the reversible fuel system 190 is operatedat a higher current density, such as at a peak current density, in theelectrolysis mode during the day time period to produce the excess fuel.

In a second preferred embodiment, the reversible fuel cell system 190operates for a longer duration in the electrolysis mode than in the fuelcell mode over the predetermined number of cycles. This difference inoperating time may be used to produce the excess fuel. For example, thesystem 190 may operate during one portion of the electrolysis mode toregenerate sufficient fuel for the entire next fuel cell mode period,and then operate for the remainder of the electrolysis mode period toproduce the excess fuel.

Examples of the second embodiment include system 190 operation wherecharging and discharging follows a day/night cycle. Such a system oftenoperates with a photovoltaic electrical energy source 100 describedabove. If the night time discharge period is shorter than the day timecharge period, a surplus or excess fuel can be generated.

Another example is where the system 190 is used to provide emergencybackup power when the electrical energy source 100 is unable to provideelectrical energy to the consumer 140. For example, the system 190 maybe used as a backup power source for a time when an electrical gridenergy source 100 stops providing electrical energy. In this example,the system 190 operates at least 90 to 99 percent of the time in theelectrolysis mode and occasionally operates in the fuel cell mode whenthe source 100 does not provide electrical energy. Thus, the system 190is used as an electrolyzer which provides fuel for non-system uses andas an emergency backup power generator. This mode of operation reducesthe cost of the emergency backup energy generation system.

If desired, the first and second embodiments may be combined, and thesystem 190 may operate in the electrolysis mode for a longer time periodand at a higher current density than in the fuel cell mode.

In a third preferred embodiment, the reversible fuel cell system 190 iselectrically connected an electrical energy source 100 which comprises arenewable energy source. Any suitable renewable energy source may beused. An excess capacity of the renewable energy source is used toprovide electrical energy to the reversible fuel cell operating in theelectrolysis mode to generate the excess fuel. Different renewableenergy sources have different types of excess capacities.

In one example, the renewable energy source 100 may comprises aphotovoltaic system, such as a solar cell array. The photovoltaic systemcontains extra capacity during a first portion, such as the first 95 to99.9 percent, of its designed lifespan. In other words, the capacity ofthe photovoltaic system decreases as the system ages during its lifespan. Thus, a photovoltaic system is often designed to provide a desiredamount of electrical energy based on the remaining second portion of itsexpected lifespan, to prevent the photovoltaic system from providing aninsufficient amount of electrical energy in the second portion of itsexpected life span. Therefore, the photovoltaic system is designed andsized to provide electrical energy in excess of that required by theconsumer and that required by the reversible fuel cell system 190 toregenerate fuel for operation in the fuel cell mode. The excess capacityof the photovoltaic system during the first portion, such as the 95 to99.9 percent, of its life span may be used to provide electrical energyto the reversible fuel cell system 190 to generate the excess fuel.

In another example, the photovoltaic system 100 is designed and sized toprovide a sufficient amount of electrical energy required by theconsumer and required by the reversible fuel cell system 190 toregenerate fuel for operation in the fuel cell mode over a predeterminednumber of day and night cycles, even if a large amount of days arecloudy. In other words, the photovoltaic system 100 is designed toprovide a sufficient amount of electrical energy in a worst case weatherscenario, such as when a predetermined percent, X, of day time periodsare cloudy. However, the worst case weather scenario occursoccasionally. Thus, when less than X percent of day time periods arecloudy, and the system 190 is fully recharged for operation in the nextfuel cell mode period or periods, the photovoltaic system 100 may beused to provide electrical energy to system 190 to generate excess fuelduring the excess sunny portions of the day time periods. For example,the photovoltaic system 100 is designed and sized to provide asufficient amount of electrical energy required by the consumer andrequired by the reversible fuel cell system 190 in the winter, when thenumber of sunny days is at a minimum and/or when the day length isshortest compared to the night length. Thus, there is no sufficientsunlight during X percent of the time during winter. The photovoltaicsystem 100 has excess capacity in the summer, when there is nosufficient sunlight during only X-Y percent of the time. Thephotovoltaic system 100 may be used to provide electrical energy tosystem 190 to generate excess fuel during Y percent of the time duringthe summer.

In another example, the renewable energy source 100 may comprise a windturbine system which is designed to provide a minimum amount ofelectrical energy at a predetermined wind speed. Such systems oftencontain rotatable blades coupled to a generator which generateselectricity when the wind rotates the blades. This system 100 containsextra capacity during periods when the wind speed exceeds apredetermined wind speed.

The wind turbine system 100 is designed and sized to provide asufficient amount of electrical energy required by the consumer andrequired by the reversible fuel cell system 190 to regenerate fuel foroperation in the fuel cell mode over a predetermined number of cycles,even if a there is no wind or the wind speed is low during a largeportion of a predetermined number of cycles. In other words, the system100 is designed to provide a sufficient amount of electrical energy in aworst case weather scenario, such as when a predetermined percent, X, ofthe time there is no wind or the wind speed is lower than desirable.However, the worst case weather scenario occurs occasionally. Thus, whenless than X percent of the time period has little or no wind and thesystem 190 is fully recharged for operation in the next fuel cell modeperiod or periods, the system 100 may be used to provide electricalenergy to system 190 to generate excess fuel during the excess windyportions of the time period. For example, if X-Y percent of the timeperiod has no wind or insufficient wind, then the system 100 may be usedto provide electrical energy to system 190 to generate excess fuelduring Y percent of time period.

In another example, the renewable energy source 100 may comprise a tidalenergy generation system which is designed to provide a minimum amountof electrical energy at a predetermined tidal force. Such a system 100contains movable members, such as plates, located under a body of water,such as a sea, ocean or lake. The movable members are connected to agenerator. The movable members are moved by the tides and the movementcauses the generator to generate electricity. This system contains extracapacity during periods when the tidal force exceeds the predeterminedtidal force.

The tidal energy generation system 100 is designed and sized to providea sufficient amount of electrical energy required by the consumer andrequired by the reversible fuel cell system 190 to regenerate fuel foroperation in the fuel cell mode over a predetermined number of cycles,even if a there is no tide or the tidal force is low during a largeportion of a predetermined number of cycles. In other words, the system100 is designed to provide a sufficient amount of electrical energy in aworst case tidal scenario, such as when a predetermined percent, X, ofthe time the tidal force is lower than desirable. However, the worstcase tidal scenario occurs occasionally. Thus, when less than X percentof the time period has insufficient tidal force and the system 190 isfully recharged for operation in the next fuel cell mode period orperiods, the system 100 may be used to provide electrical energy tosystem 190 to generate excess fuel during the excess high tidal forceportions of the time period. For example, if X-Y percent of the timeperiod has insufficient tidal force, then the system 100 may be used toprovide electrical energy to system 190 to generate excess fuel during Ypercent of time period.

In another example, the renewable energy source 100 may comprise ageothermal energy generation system which is designed to provide aminimum amount of electrical energy at a predetermined geothermalenergy. Such a system 100 uses the heat and/or steam emitted from theearth and converts the heat and/or steam into electrical energy. Thissystem contains extra capacity during periods when the geothermal energyexceeds a predetermined, worst case thermal energy supply scenario. Forexample, ambient losses for the geothermal energy differ between warmsummer ambient temperatures and cold winter ambient temperatures.

Thus, as described above, co-production of fuel in an energy storagedevice using a fuel regenerating device can be realized in any situationwhere the fuel consumed during discharge is less than the fuelregenerated during charging periods, preferably over a predeterminednumber of charge and discharge cycles. The system 190 generates fuelduring the entire electrolysis mode time period. From about 1 to about99 percent of the generated fuel, such as about 10 to about 30 percentof the generated fuel is excess fuel which may be used for non-system120 uses, while the remaining fuel may be used to operate the system 190in the fuel cell mode.

It is preferred, but not required to use the system 120 with a renewableenergy source 100 in remote locations that are not connected to thepower grid. In this case, the reversible fuel cell system 190 of theelectrical energy storage system 120 may be used to generate fuel forair, land or water vehicles. A vehicle fuel infrastructure is notrequired in this case and the system 120 may be used to supply thenecessary fuel to power the vehicles at the remote location. Forexample, the vehicles may be powered by hydrogen, methane or otherhydrocarbon fuel. Furthermore, since the system 190 may be used togenerate electricity and an environmentally clean fuel, such ashydrogen, without emission of greenhouse gases and carbon dioxide, thesystem 190 improves the environment and reduces the emission ofgreenhouse gases for transportation and other uses.

The excess generated fuel may be used for any suitable applicationoutside the system 120. For example, the excess fuel may be provided topower an airborne vehicle, such as a rocket, airplane, helicopter orblimp, a water based vehicle, such as a ship or submarine, a land basedvehicle, such as a car, truck, motorcycle, tank or train, a chemicalreaction in a chemical manufacturing process, such as a semiconductormanufacturing or chemical production process, or a heating system of abuilding, such as a commercial building, including office buildings,factories and hospitals and a residential building.

In another preferred embodiment of the present invention, theequilibrium operating temperature of the reversible fuel cell system190, such as a fuel cell stack, in the electrolysis mode is selectedindependently from the equilibrium operating temperature of the fuelcell stack in the fuel cell mode to optimize the amount and/or cost ofthe fuel produced in the electrolysis mode. Thus, in this embodiment,the equilibrium operating temperature of the fuel cell stack in theelectrolysis mode is preferably, but not necessarily different from theequilibrium operating temperature of the fuel cell stack in the fuelcell mode.

In one preferred aspect, the equilibrium operating temperature of thefuel cell stack in the electrolysis mode may be selected to optimize ormaximize the amount of fuel produced. In another preferred aspect, theequilibrium operating temperature of the fuel cell stack in theelectrolysis mode may be selected to optimize or minimize the unit costof the fuel produced. In this aspect, in certain situations, dependingon different fuel cell stack design and operating variables, one or moreminima may be observed on a curve of unit fuel cost versus amount offuel produced. The equilibrium operating temperature of the fuel cellstack in the electrolysis mode may be optimized to operate at or nearone of the minima on this curve. In another preferred aspect, theequilibrium operating temperature of the fuel cell stack in theelectrolysis mode is selected to maximize the amount of fuel producedduring some time periods, such as during periods of high fuel demand orprice, and at other time periods it is selected to minimize the fuelunit cost, such as during periods of low fuel price or demand. Thus, theequilibrium operating temperature of the fuel cell stack in theelectrolysis mode may be higher or lower than that in the fuel cell modedepending on the desired optimized condition, such as fuel amount orfuel unit cost.

Preferably, the equilibrium operating temperature of the fuel cell stackin the electrolysis mode is obtained by adjusting the current densityprovided to the fuel cell stack for a given reactant product flow rate.For example, the power management system 160 may be used to control thecurrent density provided to the fuel cell stack (i.e., the reversiblefuel cell system 190). As discussed above, the system 160 may be anysuitable control system, such as a computer or microprocessor.Alternatively, the system 160 may comprise a manual current densitycontrol system, such as a manually rotated knob, lever, dial or one ormore push buttons.

Alternatively, the equilibrium operating temperature of the fuel cellstack may be adjusted by other methods. For example, the current densitymay be held constant while the reactant product flow rate may beadjusted manually or automatically by a controller, such as by a manualor automatic valve. If desired, both the current density and thereactant product flow rate may be adjusted to select the desiredtemperature.

The selection of an optimum equilibrium operating temperature of thefuel cell stack in the electrolysis mode independent of the equilibriumoperating temperature of the fuel cell stack in the fuel cell modeprovides an operational advantage. As noted above, in order to thermallysustain the SORFC without the use of thermal storage duringelectrolysis, electrolysis should be performed above the thermal neutralvoltage. Thus, the thermal losses keep the fuel cell stack at a desiredtemperature. If the fuel cell stack is used to generate fuel for useoutside the system during electrolysis, the total amount of heatgenerated in the electrolysis mode is the same. However, some of theheat is used to generate the excess fuel rather than being lost, whichreduces the thermal losses and provides an operational cost advantagefor the fuel cell stack that is used to generate excess hydrogen.

As discussed above, the reversible fuel cell system 190 may comprise anysuitable system, such as a SORFC system, a PEM system or fuelcell/electrolyzer pairs and may be used with any suitable electricalenergy source 100 described above, such as a power grid or a renewableenergy source. A SORFC stack is the preferred reversible fuel cellsystem 190.

A single SORFC 10 operating in the electrolysis mode is shown in FIG. 4.The SORFC contains an anode electrode 11, an electrolyte 13 and acathode electrode 12. An anode gas chamber 14 is formed between theelectrolyte 13 and an anode side interconnect (not shown forsimplicity). A cathode gas chamber 15 is formed between the electrolyte13 and a cathode side interconnect (also not shown for simplicity).

A reaction product gas mixture 17 may contain primarily water ifhydrogen is used as a fuel. Alternatively, the reaction product gasmixture 17 may contain primarily water vapor and carbon dioxide if acarbon containing gas or liquid is used as a fuel. The reaction productgas mixture 17 is introduced into the cathode gas chamber 15. A directcurrent power source (not shown) is connected to the anode electrode 11and the cathode electrode 12 in such a way that when electrical currentis flowing, the anode electrode 11 takes on a positive voltage chargeand the cathode electrode 12 takes on a negative voltage charge. Whenthe electric current is flowing, the gas mixture 17 gives up oxygen ions16 to form cathode discharge mixture 19 consisting primarily of hydrogenand optionally carbon monoxide if mixture 17 contained carbon dioxide.Oxygen ions 16 transport across the electrolyte 13 under the electricalcurrent. The oxygen ions 16 are converted into the oxidant, such asoxygen gas 18 on the anode electrode 11 under the influence of theelectrical current. The oxygen gas 18 is discharged from the anodechamber 14, while the electrolysis product (e.g., hydrogen andoptionally carbon monoxide) is collected from the cathode chamber.

A single SORFC 20 operating in the fuel cell mode is shown in FIG. 5.SORFC 20 is the same as SORFC 10, except that the cathode and anodedesignations of its electrodes are reversed. Cathode electrode 21 is thesame electrode as that identified as the anode electrode 11 in FIG. 4when operating in the electrolysis mode. Anode electrode 22 is the sameelectrode as that identified as the cathode electrode 12 in FIG. 4 whenoperating in the electrolysis mode. Solid oxide electrolyte 23 is thesame electrolyte as that identified as electrolyte 13 in FIG. 4 whenoperating in the electrolysis mode. Cathode gas chamber 24 is the samegas chamber as that identified as the anode gas chamber 14 in FIG. 4when operating in the electrolysis mode. Anode gas chamber 25 is thesame gas chamber as that identified as the cathode gas chamber 15 inFIG. 4 when operating in the electrolysis mode.

A fuel gas 27 is introduced into the anode gas chamber 25. Oxygen gas 28is introduced into the cathode chamber 24. The fuel may comprisehydrogen, a hydrocarbon gas, such as methane, and/or carbon monoxide.Water may be added to the fuel if desired. An electrical fuel cell load(not shown) is applied to the SORFC 20 and the oxygen gas 28 formsoxygen ions 26 under the influence of the electrical load. Oxygen ions26 transport across the electrolyte 23 under the influence of theelectrical current. On the anode electrode 22, the oxygen ions 26combine with hydrogen and optionally carbon, if present, from gasmixture 27 to form gas mixture 29 containing water vapor and optionallycarbon dioxide, if a carbon containing gas is present in the fuel 27.Gas mixture 29 is discharged from the anode chamber and stored as thereaction product. In the process described above, the SORFC 20 has madeelectrical energy or power, which is output through its electrodes.

An optional Sabatier reactor subsystem 30 to be used when the fuelcomprises methane is shown in FIG. 6. The reactor tube 31 contains acatalyst, such as a platinum family metal on an alumina support.Preferably, the catalyst comprises ruthenium. A gas mixture 32consisting primarily of hydrogen and carbon monoxide is introduced intoreactor tube 31 and contacts the catalyst therein. The gas mixture 32undergoes an immediate exothermic reaction and produces gas mixture 33consisting primarily of methane and water vapor. Gas mixture 33 is thendischarged from the reactor tube 31. When the Sabatier reactor is usedwith the SORFC 10 operating in the electrolysis mode, the hydrogen andcarbon monoxide discharge mixture 19/32 is provided from the SORFC intothe Sabatier reactor 30.

Because the reaction within reactor tube 31 is highly exothermic, a heatexchanger 34 located in or adjacent to tube 31 is used to capture thegenerated heat. Gas mixture 35, consisting primarily of carbon dioxideand water, flows through heat exchanger 34 to absorb the exothermicreaction heat. When the Sabatier reactor is used with the SORFC 10operating in the electrolysis mode, the water vapor and carbon dioxideinlet mixture 17/35 is heated in the Sabatier reactor by the reaction ofthe outlet or discharge mixture 19/32. The water vapor and carbondioxide inlet mixture 17/35 is then provided from the Sabatier reactorinto the SORFC 10.

The SORFC system 50 of a preferred embodiment operating in a fuel cellmode is shown of FIG. 7 as a simplified schematic. The system 50 asshown operates with methane as a fuel. However, if desired, a hydrogenfuel may be used instead, as discussed above. A single SORFC 20previously shown in FIG. 5 as a cross section operating in the fuel cellmode is shown again in FIG. 7. While a single SORFC is shown, it shouldbe understood that the system 50 contains a plurality of SORFC stacks. Ahydrogen recovery unit 51 transfers hydrogen gas from within a first gasmixture stream into a second gas stream. The hydrogen recovery unit 51can be a device which recovers hydrogen based on absorption/adsorptionprocesses or based on an electrochemical proton exchange process. Theelectrochemical exchange process is preferred.

An enthalpy recovery unit 52 transfers water vapor from first gas streamto a second gas stream. The enthalpy recovery unit 52 can be a devicewhich transfers water vapor based on cyclic desiccant beds or a rotatingdesiccant wheel. The desiccant wheel (i.e., “enthalpy wheel”) ispreferred. An optional purge valve 53, such as a normally closed poweredopen solenoid valve may be used if pure oxygen is used. A heat exchanger54 is a counter flow gas-gas heat exchanger. The SORFC power output,such as output electrode(s), is connected to a power distributionsystem. The oxidizer (i.e., oxygen or air) enters the system 50 throughthe oxidizer inlet or conduit 55, while the fuel enters the systemthrough the fuel inlet or conduit arrangement 56/57. The fuel exhaustexits through conduit arrangement 58/59.

A method of operating the system 50 in the fuel cell mode is nowdescribed. Within the SORFC system 50 shown in FIG. 7, oxidizer, such aspure oxygen reactant gas from an oxygen storage vessel, such as a liquidoxygen tank, or air, is delivered to the cathode chamber of SORFC 20through inlet conduit 55. If oxygen reactant is highly pure, then it isnormally dead headed within the cathode chamber of SORFC 20. However,even the purest of gases will include trace non reactant gas species. Asa result the cathode chamber of SORFC 20 should be occasionally purgedof these non reactant species. Oxygen purge valve 53 is used toaccomplish this purging.

High purity hydrocarbon inlet stream, such as a methane stream, isintroduced into the SORFC system 50 from a hydrocarbon storage vessel,such as a tank (not shown for clarity), through conduit 56 into thehydrogen recovery unit 51. As noted above, a hydrogen fuel inlet streammay be used instead. Within the hydrogen recovery unit 51, hydrogen gasis transferred from the fuel exhaust outlet stream in conduit 58 intothe methane stream. This hydrogen supports a uniform methane reformationprocess within the anode chamber of SORFC 20. The methane and hydrogenmixture next is introduced into the enthalpy recovery unit 52, where aportion of the water vapor is transferred from the fuel exhaust outletstream in conduit 58 into the methane and hydrogen inlet stream.Preferably, the enthalpy recovery unit also transfers heat from theoutlet stream to the inlet stream. From the enthalpy recovery unit 52,the methane, hydrogen and water vapor mixture is introduced into theheat exchanger 54, where the gas mixture temperature is increased nearto the operational temperature of 600 C. to 1000 C. using the hightemperature waste heat from the outlet stream in conduit 58. From heatexchanger 54, the hot mixture of methane, hydrogen, and water vapor isdelivered to the anode chamber of SORFC 20 through conduit 57. Somesteam reformation of the methane will occur in the heat exchanger 54 andconduit 57 but the amount is suppressed by the existence of thehydrogen. The completion of the steam reforming of the methane isaccomplished in the anode chamber of the SORFC 20.

Within the anode chamber of the SORFC 20, the steam reforming of methaneand the oxidation of carbon and hydrogen in the fuel cell reactionsconverts the discharged gas mixture (i.e., fuel exhaust) in conduit 58to carbon dioxide, additional water vapor, and excess hydrogen. Ifhydrogen rather than methane is used as a fuel, then no carbon dioxideis produced. The discharged gas mixture in conduit 58 passes throughheat exchanger 54, releasing waste heat, and then through the enthalpyrecovery unit 52 to supply a portion of the water vapor to support theinput methane reformation. The discharged gas mixture in conduit 58 isthen directed to the hydrogen recovery unit 51 where virtually all buttrace quantities of the hydrogen is transferred to the inlet fuelstream. Using the preferred electrochemical proton exchange process asthe hydrogen recovery unit 51, provides an exact measure of the hydrogencontent within the discharged gas mixture in conduit 58 which is used toadjust the input methane flow rate. The outlet mixture in conduit 59from hydrogen recovery unit 51 contains only carbon dioxide and waterwhich are stored separately (not shown).

The SORFC system 60 of a preferred embodiment operating in anelectrolysis mode is shown of FIG. 8 as a simplified schematic. A singleSORFC 10 previously shown in FIG. 4 as a cross section operating in theelectrolysis mode is shown again in FIG. 8. The hydrogen recovery unit51 transfers hydrogen gas from within a first gas mixture stream into asecond gas stream. The hydrogen recovery unit 51 can be a device whichrecovers hydrogen based on absorption/adsorption processes or based onan electrochemical proton exchange process. The electrochemical exchangeprocess is preferred.

If methane is used as a fuel, then the system 60 also includes theSabatier reactor subsystem 30, described with respect to FIG. 6, whichconverts carbon monoxide and hydrogen into methane and water vapor. Ifmethane is used as a fuel, then the reactant product comprising carbondioxide and water enter the system 60 through inlet or conduit 61, whichmay be the same or different than conduit 56, shown in FIG. 7. Ifhydrogen is used as a fuel, then water is used as a reactant product.The generated oxygen exits through outlet or conduit 65, while themethane and water exit through outlet or conduit arrangement 63/64.Conduits 63/64 and 65, respectively, may be the same or differentconduits as conduits 58/59 and 55, respectively, shown in FIG. 7.

Thus, the system 60 operating in the electrolysis mode is the samesystem as system 50 operating in the fuel cell mode, except that theinlet and outlet streams are steered through the optional Sabatierreactor subsystem 30 instead of through the heat exchanger 54 and theenthalpy recovery unit 52, which remains inactive in the electrolysismode. The inlet and outlet streams may be steered using valves andparallel conduits (not shown for clarity). Furthermore, the electrodedesignations in the SORFC 10 of system 60 are reversed compared to SORFC20 of system 50, as explained in detail with respect to FIGS. 1 and 2above.

A method of operating the system 60 in the electrolysis mode is nowdescribed. If methane is used as a fuel, then carbon dioxide and waterare introduced into the SORFC system 60 through conduit 61 into hydrogenrecovery unit 51. Carbon dioxide may be introduced from a carbon dioxidestorage vessel or from a conduit. If hydrogen is used as a fuel, thenthe carbon dioxide is omitted. Within the hydrogen recovery unit 51,hydrogen gas is transferred from the outlet stream in conduit 63 intothe carbon dioxide and water inlet stream. This extra hydrogeneventually assures that all the carbon bearing gases are converted intomethane within the Sabatier reactor subsystem 30. The carbon dioxide,water, and hydrogen inlet mixture next is introduced into the Sabatiersubsystem 30 heat exchanger where it is heated by the exothermicreaction. From the Sabatier subsystem 30, the carbon dioxide, hydrogenand water vapor mixture is delivered to the cathode chamber of SORFC 10through conduit 62. Within the cathode chamber of SORFC 10, the carbondioxide and water vapor are reduced by electrolysis to carbon monoxideand hydrogen. Excess water and some unreacted carbon dioxide will bedischarged from the cathode chamber of SORFC 10 along with the carbonmonoxide and hydrogen through conduit 63.

The discharged gas mixture in conduit 63 passes through the Sabatiersubsystem 30 to convert all the carbon oxides to methane and water withthe excess hydrogen. If hydrogen is used as a fuel, then the Sabatiersubsystem 30 is omitted and the discharged water vapor comprises thereactant product. The discharged gas mixture in conduit 63 is thendirected to the hydrogen recovery unit 51 wherein virtually all buttrace quantities of the hydrogen is transferred to the inlet carbondioxide and water stream. Using the preferred electrochemical protonexchange process as the hydrogen recovery unit 51, provides an exactmeasure of the hydrogen content within the discharged gas mixture inconduit 63 which is used to adjust the input carbon dioxide flow rate.The outlet mixture in conduit 64 from hydrogen recovery unit 51 containsonly methane and water which are stored separately (not shown). Ifdesired, the water may be discharged and fresh water from a water pipemay be used for the SORFC reactions.

In the meantime, pure oxygen gas is generated in the SORFC 10 anodeduring the electrolysis process. The oxygen is discharged from the SORFC10 anode through conduit 65 and on to discharge, direct metabolic useand/or to liquefied storage (not shown).

The Sabatier reactor which generates methane is advantageous because itoperates at a temperature of about 400-900° C. degrees, which is asuitable temperature for heating the inlet stream being provided intothe SORFC to or near to a desired SORFC operating temperature. However,other reactors which generate hydrocarbon gases other than methane maybe used instead of the Sabatier reactor.

For example, reactors which convert an exhaust gas which containshydrogen and carbon oxides, such as carbon monoxide and/or carbondioxide, and optionally water, to methanol may be used instead. Themethanol reactors typically, but not necessarily, contain a coppercatalyst which converts hydrogen, carbon oxides and/or water vapor tomethanol. These reactors may be catalyst bed type reactors, such as ARCreactors, quench converters, tube cooled converters, isothermal reactorswhere a continuous catalyst bed surrounds a spiral wound heat exchanger,and other suitable reactor types.

The following exothermic reactions are involved in the synthesis ofmethanol: CO+2H₂═CH₃OH; CO₂+3H₂═CH₃OH and CO+H₂O═CO₂+H₂. The use of aSORFC operating in the electrolysis mode to generate methanol isadvantageous because the SORFC exhaust gas contains a similarcomposition to synthesis gas that is used as a source gas for methanolproduction. The synthesis gas is usually specially prepared in aseparate catalytic steam reforming of natural gas in conventionalmethanol synthesis process.

If desired, additional reactors may be present downstream of theSabatier or methanol reactors to further purify the methane or methanolif desired. Alternatively, the additional reactors may be used toconvert methane or methanol to other hydrocarbon gases, such as ethane,propane, octane, formic acid, formaldehyde and/or other suitablehydrocarbon gases. These hydrocarbon gases may be used as a fuel for theSORFC in the fuel cell mode and/or may be removed from the SORFC systemfor other use, sale or storage. Thus, the SORFC system may be used togenerate various hydrocarbon fuels for storage or sale when the systemis not generating power in the fuel cell mode. Alternatively, suitablereactors may be used to convert the hydrogen and carbon oxide containingSORFC electrolysis mode exhaust to the other hydrocarbon gases, such asethane, propane, octane, formic acid, formaldehyde and/or other suitablehydrocarbon gases.

The SORFC systems described herein may have other embodiments andconfigurations, as desired. Other components, such as fuel side exhauststream condensers, heat exchangers, heat-driven heat pumps, turbines,additional gas separation devices, hydrogen separators which separatehydrogen from the fuel exhaust and provide hydrogen for external use,fuel preprocessing subsystems, fuel reformers and/or water-gas shiftreactors, may be added if desired, as described, for example, in U.S.application Ser. No. 10/300,021, filed on Nov. 20, 2002, in U.S.Provisional Application Ser. No. 60/461,190, filed on Apr. 9,2003, andin U.S. application Ser. No. 10/446,704, filed on May 29, 2003 allincorporated herein by reference in their entirety.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

1. An electrochemical system, comprising: a reversible fuel cell system which generates electrical energy and reactant product from fuel and oxidizer in a fuel cell mode and which generates the fuel and oxidant from the reactant product and the electrical energy in an electrolysis mode; a reactant product delivery device which is adapted to supply the reactant product to the reversible fuel cell system operating in the electrolysis mode, in addition to or instead of the reactant product generated by the reversible fuel cell system in the fuel cell mode; and a fuel removal device which is adapted to remove the fuel generated by the reversible fuel cell system operating in the electrolysis mode from the electrochemical system.
 2. The system of claim 1, further comprising a fuel storage device and a reactant product storage device.
 3. The system of claim 2, wherein: the reactant product delivery device comprises a reactant product conduit which is connected to the reactant product storage device to provide the reactant product to the reactant product storage device from outside of the electrochemical system; and the fuel removal device comprises a fuel conduit which is connected to the fuel storage device to deliver fuel from the fuel storage device outside of the electrochemical system.
 4. The system of claim 3, wherein: reactant product conduit comprises a water conduit; and the fuel conduit comprises a hydrogen conduit.
 5. The system of claim 3, wherein: reactant product conduit comprises a water conduit and a carbon dioxide conduit; and the fuel conduit comprises a methane conduit.
 6. The system of claim 3, further comprising a power management subsystem which is adapted to control an operation of the reversible fuel cell system and to provide electrical energy to and from the reversible fuel cell system.
 7. The system of claim 3, wherein the reversible fuel cell system comprises a stack of plurality of reversible fuel cells, which generate the electrical energy in the fuel cell mode and which generate the fuel in the electrolysis mode.
 8. The system of claim 7, wherein the reversible fuel cells comprise a stack of solid oxide regenerative fuel cells.
 9. The system of claim 7, wherein the reversible fuel cells comprise PEM fuel cells.
 10. The system of claim 1, wherein the reversible fuel cell system comprises a fuel cell which generates electrical energy and an electrolyzer cell which generates fuel.
 11. The system of claim 1, further comprising a renewable energy source electrically connected to the reversible fuel cell system, such that an excess capacity of the renewable energy source is used to provide electrical energy to the reversible fuel cell system operating in the electrolysis mode to generate excess fuel to be provided to the fuel removal device.
 12. An electrochemical system, comprising: a reversible fuel cell system which generates electrical energy and reactant product from fuel and oxidizer in a fuel cell mode, and which generates the fuel and oxidant from the reactant product and the electrical energy in an electrolysis mode; a second means for providing excess reactant product to the reversible fuel cell system operating in the electrolysis mode from outside the electrochemical system, in addition to or instead of the reactant product generated by the reversible fuel cell system in the fuel cell mode, such that fuel in excess of fuel required to operate the reversible fuel cell system in the fuel cell mode is generated in the electrolysis mode over a predetermined number of operating cycles; and a third means for removing the excess fuel generated by the reversible fuel cell system operating in the electrolysis mode from the electrochemical system.
 13. The system of claim 12, further comprising a renewable energy source electrically connected to the reversible fuel cell system, such that an excess capacity of the renewable energy source is used to provide electrical energy to the reversible fuel cell system operating in the electrolysis mode to generate the excess fuel to be provided to the third means.
 14. The system of claim 12, wherein the reversible fuel cell system is electrically connected to a fourth means for generating electrical energy from sun light, for providing the electrical energy to the reversible fuel cell system during daytime periods to generate fuel, and for allowing the reversible fuel cell system to generate electrical energy during night time periods, wherein night time periods are shorter than day time periods.
 15. The system of claim 12, wherein the reversible fuel cell system is electrically connected to a fifth means for generating renewable electrical energy and for using excess capacity to provide electrical energy to the reversible fuel cell system operating in the electrolysis mode to generate the excess fuel.
 16. The system of claim 15, wherein the fifth means is a means for generating electrical energy from sun light and for using the excess capacity during a first half of its designed lifespan.
 17. The system of claim 15, wherein the fifth means is a means for generating electrical energy from wind and for using the excess capacity during periods when wind speed exceeds a predetermined wind speed required to generate a desired amount of electrical energy.
 18. The system of claim 15, wherein the fifth means is a means for generating electrical energy from tidal force and for using the excess capacity during periods when the tidal force exceeds a predetermined tidal force required to generate a desired amount of electrical energy.
 19. The system of claim 12, wherein the third means is a means for providing the fuel removed from the reversible fuel cell system into an airborne vehicle, a water based vehicle, a land based vehicle, a chemical reaction in a chemical manufacturing process, or a heating system of a building containing the first means.
 20. A method of operating an electrochemical system containing a reversible fuel cell system, comprising: cyclically operating the reversible fuel cell system in a fuel cell mode to generate electrical energy and reactant product from fuel and oxidizer and in an electrolysis mode to generate the fuel and oxidant from the reactant product and the electrical energy; providing excess reactant product to the reversible fuel cell system operating in the electrolysis mode from outside the reversible fuel cell system, in addition to or instead of the reactant product generated by the reversible fuel cell system in the fuel cell mode, such that fuel in excess of fuel required to operate the reversible fuel cell system in the fuel cell mode is generated in the electrolysis mode over a predetermined number of operating cycles; and removing the excess fuel generated by the reversible fuel cell system operating in the electrolysis mode from the electrochemical system.
 21. The method of claim 20, wherein the reversible fuel cell system generates more fuel in the electrolysis mode than it consumes in the fuel cell mode over the predetermined number of cycles.
 22. The method of claim 21, wherein the reversible fuel cell system operates at a higher current level in the electrolysis mode than in the fuel cell mode over the predetermined number of cycles.
 23. The method of claim 22, wherein: the reversible fuel cell system is electrically connected to a photovoltaic energy generation system which provides electrical energy to the reversible fuel cell system during daytime to generate fuel; the reversible fuel cell system generates electrical energy during night time; and the night time load on the reversible fuel cell system at least during a portion of the night time period is lower than a peak load that the reversible fuel cell system is capable of providing.
 24. The method of claim 22, wherein the reversible fuel system is operated at a load lower than a peak load that the reversible fuel cell system is capable of providing at least during a portion of a time that the reversible fuel cell system operates in the fuel cell mode.
 25. The method of claim 21, wherein the reversible fuel cell system operates for a longer duration in the electrolysis mode than in the fuel cell mode over the predetermined number of cycles.
 26. The method of claim 25, wherein: the reversible fuel cell system is electrically connected to a photovoltaic energy generation system which provides electrical energy to the reversible fuel cell system during daytime to generate fuel; the reversible fuel cell system generates electrical energy during night time; and night time periods are shorter than day time periods.
 27. The method of claim 21, wherein the reversible fuel cell is electrically connected to a renewable energy source, such that an excess capacity of the renewable energy source is used to provide electrical energy to the reversible fuel cell operating in the electrolysis mode to generate the excess fuel.
 28. The method of claim 27, wherein the renewable energy source comprises a photovoltaic system which contains the excess capacity during a first half of its designed lifespan.
 29. The method of claim 27, wherein the renewable energy source comprises a wind turbine system which is designed to provide a minimum required amount of electrical energy at a predetermined minimum wind speed and which contains the excess capacity during periods when the wind speed exceeds the predetermined minimum wind speed.
 30. The method of claim 27, wherein the renewable energy source comprises a tidal energy generation system which is designed to provide a minimum required amount of electrical energy at a predetermined minimum tidal force and which contains the excess capacity during periods when the tidal force exceeds the predetermined minimum tidal force.
 31. The method of claim 20, wherein the step of providing the excess reactant product comprises providing the excess reactant product to the reversible fuel cell system operating in the electrolysis mode from outside the electrochemical system in addition to a stored reactant product generated by the reversible fuel cell system in the fuel cell mode.
 32. The method of claim 20, wherein the step of providing the excess reactant product comprises providing the excess reactant product to the reversible fuel cell system operating in the electrolysis mode from outside the electrochemical system instead of the reactant product generated by the reversible fuel cell system in the fuel cell mode.
 33. The method of claim 20, further comprising storing the fuel and the reactant product produced by the reversible fuel cell system.
 34. The method of claim 33, wherein: at least a portion of the stored fuel is removed from the reversible fuel cell system through a fuel conduit; and at least a portion of the reactant product is provided to the reversible fuel cell system from outside the electrochemical system through a reactant product conduit.
 35. The method of claim 33, wherein at least a portion of the stored fuel is removed from the electrochemical system by removing a fuel storage vessel from the electrochemical system.
 36. The method of claim 20, wherein: the reactant product comprises water; and the fuel comprises hydrogen.
 37. The method of claim 20, wherein: the reactant product comprises water and carbon dioxide; and the fuel comprises methane.
 38. The method of claim 20, wherein: the reversible fuel cell system comprises a stack of a plurality of reversible fuel cells, which generate electrical energy in the fuel cell mode and which generate fuel in the electrolysis mode; and an equilibrium operating temperature of the fuel cell stack in the electrolysis mode is selected independently from an equilibrium operating temperature of the fuel cell stack in the fuel cell mode to optimize at least one of an amount of fuel produced in the electrolysis mode and a unit cost of the fuel produced in the electrolysis mode.
 39. The method of claim 20, wherein the reversible fuel cell system comprises a stack of solid oxide regenerative fuel cells.
 40. The method of claim 20, wherein the reversible fuel cell system comprises a plurality of PEM fuel cells.
 41. The method of claim 20, wherein the reversible fuel cell system comprises a fuel cell which generates electrical energy and an electrolyzer cell which generates fuel.
 42. The method of claim 20, further comprising providing the fuel removed from the electrochemical system into an airborne vehicle, a water based vehicle or a land based vehicle.
 43. The method of claim 20, further comprising providing the fuel removed from the electrochemical system into a chemical reaction in a chemical manufacturing process.
 44. The method of claim 20, further comprising providing the fuel removed from the electrochemical system into an heating system of a building containing the reversible fuel cell system.
 45. An electrochemical system, comprising: a first means for cyclically operating in a fuel cell mode to generate electrical energy and reactant product from fuel and oxidizer and in an electrolysis mode to generate the fuel and oxidant from the reactant product and the electrical energy; a second means for providing excess reactant product to the first means operating in the electrolysis mode from outside the electrochemical system, in addition to or instead of the reactant product generated by the first means in the fuel cell mode, such that fuel in excess of fuel required to operate the first means in the fuel cell mode is generated in the electrolysis mode over a predetermined number of operating cycles; and a third means for removing the excess fuel generated by the first means operating in the electrolysis mode from the electrochemical system.
 46. The system of claim 45, wherein the first means generates more fuel in the electrolysis mode than it consumes in the fuel cell mode over the predetermined number of cycles.
 47. The system of claim 46, wherein the first means is a means for operating at a higher current level in the electrolysis mode than in the fuel cell mode over the predetermined number of cycles.
 48. The system of claim 47, wherein: the first means is electrically connected to a fourth means for generating electrical energy from sun light and for providing the generated electrical energy to first means during daytime to generate fuel; and the first means is a means for generating electrical energy during night time, such that a night time load on the first means at least during a portion of the night period is lower than a peak load that the first means is capable of providing.
 49. The system of claim 47, wherein the first means is a means for operating at a load lower than a peak load that the first means is capable of providing at least during a portion of a time that the first means operates in the fuel cell mode.
 50. The system of claim 46, wherein the first means is a means for operating for a longer duration in the electrolysis mode than in the fuel cell mode over the predetermined number of cycles.
 51. The system of claim 50, wherein: the first means is electrically connected to a fourth means for generating electrical energy from sun light and providing the electrical energy to the first means during daytime periods to generate fuel; the first means is a means for generating electrical energy during night time periods, wherein night time periods are shorter than daytime periods.
 52. The system of claim 45, wherein the first means is electrically connected to a fifth means for generating renewable electrical energy and for using excess capacity to provide electrical energy to the first means operating in the electrolysis mode to generate the excess fuel.
 53. The system of claim 52, wherein the fifth means is a means for generating electrical energy from sun light and for using the excess capacity during a first half of its designed lifespan.
 54. The system of claim 52, wherein the fifth means is a means for generating electrical energy from wind and for using the excess capacity during periods when the wind speed exceeds a predetermined minimum wind speed required to generate a desired amount of electrical energy.
 55. The system of claim 52, wherein the fifth means is a means for generating electrical energy from tidal force and for using the excess capacity during periods when the tidal force exceeds a predetermined minimum tidal force required to generate a desired amount of electrical energy.
 56. The system of claim 45, wherein the second means for providing excess reactant product comprises a means for providing excess reactant product to the first means operating in the electrolysis mode from outside the electrochemical system in addition to a stored reactant product generated by the first means in the fuel cell mode.
 57. The system of claim 45, wherein the second means for providing excess reactant product comprises a means for providing excess reactant product to the first means operating in the electrolysis mode from outside the electrochemical system instead of the reactant product generated by the first means in the fuel cell mode.
 58. The system of claim 45, further comprising a sixth means for storing the fuel produced by the first means and a seventh means for storing the reactant product produced by the first means.
 59. The system of claim 45, wherein: the reactant product comprises water; and the fuel comprises hydrogen.
 60. The system of claim 45, wherein: the reactant product comprises water and carbon dioxide; and the fuel comprises methane.
 61. The system of claim 45, wherein the third means is a means for providing the fuel removed from the first means into an airborne vehicle, a water based vehicle, a land based vehicle, a chemical reaction in a chemical manufacturing process, or a heating system of a building containing the first means.
 62. The system of claim 45, further comprising an eighth means for selecting an equilibrium operating temperature of the first means in the electrolysis mode independently from an equilibrium operating temperature of the first means in the fuel cell mode to optimize at least one of an amount of fuel produced in the electrolysis mode and a unit cost of the fuel produced in the electrolysis.
 63. A method of operating a reversible fuel cell system, comprising cyclically operating the reversible fuel cell system in a fuel cell mode to generate electrical energy and reactant product from fuel and oxidizer at a first equilibrium temperature and in an electrolysis mode to generate the fuel and oxidant from the reactant product and the electrical energy at a second equilibrium temperature, wherein the first equilibrium temperature is different from the second equilibrium temperature.
 64. The method of claim 63, further comprising selecting the first equilibrium operating temperature independently from the second equilibrium operating temperature to optimize at least one of an amount of fuel produced in the electrolysis mode and a unit cost of the fuel produced in the electrolysis mode.
 65. The method of claim 64, wherein the first equilibrium operating temperature is selected to maximize the amount of fuel produced in the electrolysis mode.
 66. The method of claim 64, wherein the first equilibrium operating temperature is selected to minimize the unit cost of the fuel produced in the electrolysis mode.
 67. The method of claim 64, wherein the first operating temperature is selected by adjusting an amount of current density provided to the reversible fuel cell system.
 68. The method of claim 64, wherein the first operating temperature is selected by adjusting an amount of the reactant product provided to the reversible fuel cell system.
 69. The method of claim 64, further comprising: providing excess reactant product to the reversible fuel cell system operating in the electrolysis mode from outside the reversible fuel cell system, in addition to or instead of the reactant product generated by the reversible fuel cell system in the fuel cell mode, such that fuel in excess of fuel required to operate the reversible fuel cell system in the fuel cell mode is generated in the electrolysis mode over a predetermined number of operating cycles; and removing the excess fuel generated by the reversible fuel cell system operating in the electrolysis mode from an electrochemical system in which the reversible fuel cell system is located.
 70. The method of claim 69, wherein: the reversible fuel cell system generates more fuel in the electrolysis mode than it consumes in the fuel cell mode over the predetermined number of cycles; and the reversible fuel cell system comprises a stack of solid oxide regenerative fuel cells. 